EFFECT OF SEED TUBER SIZE AND PLANTING SPACE ON GROWTH , MELD AND TUBER SIZE DISTRIBUTION OF POTATO ( SOLANW TUBEROSUM ) IN IRRIGATED RED-YELLOW LATOSOLS OF THE DRY

Abstract : Effect of seed tuber size and planting space on tuber yield and tuber size of potato (S.tuberosum) was studied in red-yellow latosols of dry zone under irrigation. It was found that the number of stems and tubers produced per plant significantly increased with increasing seed tuber size and planting space. The numbers of stems and tubers per plant were significantly associated. Narrow spacing increased the yield per hectare but decreased the yield per plant: Production of large size tubers were not influenced by the size of seed tubers planted. However increasing planting space increased the production of large size tubers. Because of the high seed cost, income may be optimized if the seed tubers are sorted according to size and planted in 6Ox20cm, 6Ox30cm, and 6Ox50cm spacings respectively.


INTRODUCTION
The potato (S.tuberosum) is one of humanity's most valuable food crops.It ranks fourth in the world in production after rice, wheat and maize.In dollar value it is fifth in developing countries after rice, wheat, maize and cassava.'In the yield of edible,energy and proteins per hectare per day, potato is near the top ofthe:list. of major world'food crops.'In recent years the growth rate of potato production in developing countries has exceeded that of most other food crops.' The potato gives a very high income per day compared to other crops in Sri Lanka.On average Rs.200,000 can be obtained as net income from one hectare in three months time (personal communications and farm records).In Sri Lanka, and in particular in the dry zone,the cost of seed tubers accounts for 50% of the total variable cost of production (personal communication and farm records).Because of variable returns, only few affluent farmers specialize in potato cultivation.Potato was first introduced in the dry zone in early 1960's in the Jaffna district.It is now cultivatedin many parts ofthe dry zone areas during Maha season from November to March.Small and medium size seed tubers are prefered by farmers engaged in small scale cultivation to reduce seed costs.However, the optimum spacing for different size of seed tubers and their effects on growth, yield and tuber size distribution have not been studied under dry zone irrigated conditions.I t is reported that planting large size seed tubers increased the tuber number and yield per plant over small size tubers.*"The optimum spacings, for different sizes of seed tubers that maximise tuber yield under imgated dry zone conditions are reported here.

METHODS AND MATERIALS
The field experiments were conducted a t the Faculty ofAgriculture, Kilinochchi in red-yellow latosols diiring Maha 1992193 under irrigation.The experiment was laid out in a split-plot design with seed tuber size in main plot and planting space in subplot.Three different size seed tubers with diameters 15-30mm (small), 30-45mm (medium), 45-55mm (large) with three spacings 60&0cm,60x30cm and 6Ox50cm were tested.Desiree cultivars of 3 112 months were used.Seed tubers were sprouted to 5mm by reducinglight before planting to obtain uniform and quick germination.
Fertilizer was applied as recommended by the Department of Agriculture.viz.52.5 kgN,110 kg P,O, and 45 kg&O per hectare applied basally and afurther 52.5 kg N and 45 kg &O after 30 days.Growth parameters and yield components recorded were the number of stems, leaf area index(LAI), number of tubers, tuber size distribution and tuber weight.Analysis of variance for split plot design was performed and least significant difference computed for different mean comparisons.

Number of stems and leaf area index (LAI)
Tubers germinated within ten days.Number of stems per tuber were recorded a t tuber initiation stage which occured 15 days after germination(Tab1e 1): I t was found that the number of stems produced per tuber increased with increasing tuber size and spacing.Large size seed tubers produced more stems than small ones.Large and medium size seed tubers significantly increased stem numbers over small size seed tubers.Number of stems produced by seed tubers planted a t 60 x 30 cm spacing was significantly higher a t a spacing 60 x 20 cm.However differences between 60 x 30cm and 60 x 50cm spacings were not significant.This is in accordance with earlier findings.'Positive linear regression was found between the number of stems and number of tubers produced per plht(r=0.977++).Leaf area index a t tuber initiation stage was not influenced by seed tuber size.I t was found that LA1 decreased with increasing planting space a t tuber initiation time.

Tuber number and yield
Numbei .of.tubersper plant increased with' increasing seed tuber size and planting .space.Large size seed tubers produced significantly more number of .tubers'oversmall able 2).Wiersema6 found a similar pattern in his field study.planting space.Tuber number per plot increasedwithinrreasing seed tuber size.However the difference in tuber numbers between small and medium sized seed tubers were not significant.

Number of tubers produced per plot significantly reduced with increasing
Narrow spacing increased the hectare yield and decreased the yield per plant.The highest yield was obtained with large size seed tubers (45 -55 cm) planted in narrow spacing (60 x 20cm) (Table 3).However the combination of large size seed tubers and narrow spacing produced many small size tubers of low market value.In this combination, the plant height increased and lodged due to high plant population.

Tuber size distribution
Tuber size distribution was not significantly influenced by different size of seed tubers (Table 4).The production of large size tubers were found to be more than 50% in all treatments.However increasing planting space increased the population of large size tubers.

Seed tuber cost and income
The only variable cost considered in this experiment is the cost of seed tubers.The costs of other inputs, losses due to pests, disease and harvesting costs have not been taken into consideration in the following calculation.Cost of seed tubers, total income and the income after deducting the seed tuber cost are    5).The income level of different tuber size indicates that the highest income with small, medium and large size seed tubers can be obtained a t 60 x 20cm, 60 x 30cm and 60 x 50cm spacingrespectively.However, large size seed tubers planting in narrow spacing (60 x 20cm) is not recommended because of the small tubers produced and lodging.

DISCUSSION
The marked reduction in LA1 a t 60 x 50cm spacing may be attributed to the large land area the plants occupied Number of tubers produced per plant was less with small seed tubers but it produced more large tubers.This may be because all the photo assimilates are translocated to few tubers and they increase in size.When the photo assimilates are distributed among a larger number of tubers, the percentage of large tubers is reduced.Analysis of variance performed for tuber yield and other important agronomic characters failed to h d interaction between tuber size and planting space.
The results suggest varying planting space for different sizes of seed tubers to obtain optimum economic yield.','Based on yield, tuber size distribution and other agronomic characters, the cost of seed tubers, market preference and price of yield of different size tubers, the spacing of 60 x 20cm, 60 x 30cm, and 60 x 50 cm is recommended for small, medium and large size seed thbers respectively.Farmers, instead of planting different sizes of seed tubers using the same planting distance, can separate the seed tubers according to small, medium and large sizes and use the recommended spacing for each seed tuber size to maximize their income.

Table 4 :-
Effect of seed tuber size and planting space on the production of different size tubers.Percentage of large tubers (45

Table 6 : Seed cost and income
. Attaining optimum LA1 a t early stage of growth is necessary to intercept maximum light energy."'Khurana and Mc Lared6 found that light interception by a potato crop canopy increased linearly with increasing LA1 up to 2.25.Beyond 2.25 light interception increased a t decreasing rate until a LA1 of 4.0 was attained, a t which point about 95% ofthe incomingradiation was intercepted.Increased number of stems per plant also increases the number of stolons which finally develop as tubers.This indicates a positive linear relationship between the number of stems and number of tubers developed per plant.Increased tuber yield per hectare a t closer space may be attributed to high plant population.Sidhu et.al.' reported that high yield could be obtained a t closer spacing.